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Why are the character keys on the Flight Management Computer in alphabetical order?
Regardless of which aircraft type and manufacturer, the character keys on any Flight Management Computer are always in alphabetical order, see image:

From a human factors view, this is sub-optimal, so why? Where does this order come from?
From chats with pilots I learned that you get used to it easily and can type quite fast with this alphabetical layout, but still, we are all used to the QWERTY or QWERTZ layout...

Not always every single aircraft, though most do. The A380 has QWERTY keyboards on its FMC and another pair of QWERTY keyboards on the fold out tables.

I don't have a source for this (or I'd post an answer), but I'd imagine size is a primary constraint for why it largely hasn't changed. Once someone started building the things the size they are, it's clear 10 reasonably-sized keys across won't fit. Short of a major cockpit redesign, as clearly happened with the A380 (and the 787, which is still alphabetical), the form factor constrains the choice of keyboard layout.

"we are all used to the QWERTY or QWERTZ layout..." or AZERTY. But which one? Universally there is no one standard, so whichever of these 3 (and others) is chosen, it will be wrong for some users. So the only standard left is simple alphabetic.
Another thing to consider is that the QWERTY-types of layout were chosen to deliberately slow down typists on mechanical typewriters. This slow-down is not necessary on an electronic piece of kit, so the pure-alph format is quite fast once you're used to it.

"Slow down" is not exactly the thing I believe. The reason was to avoid consecutive neighbouring letters to avoid hammer collisions.

@ymb1: To prevent hammer collisions, until IBM re-invented THE typewriter.

QWERTY layout indeed spreads letters so that most likely the next letter will be typed with the different finger (English text assuming). This is still very good for fast typing as you can start reaching for the next key before the current  keystroke is finished. Who wants to type with one finger only really needs completely different layout with frequent combinations on adjacent letters rather than spread but on a long run will not be faster.

we are all used to the QWERTY or QWERTZ layout...

Now we all have computers and most of them come with QWERTY keyboard. However in 70's when the early FMS were designed, computers were mainly in research institutions and big companies and most pilots would never use one (though their dispatch might have had some by that point). So most people would only see the layout on a typewriter and average pilot didn't need that either. So most pilots probably were not used to the layout back when it was created.
And since then, since some pilots were already used to the previous design, it was always more useful to stick with that design.

The placement and form factor of these keyboards is not suitable for applying a classic touch-typing technique.
To touch type as professionally trained, a keyboard must be in front of the typist, equally accessible with both hands, with limited number of rows (palms must stay close to the "standard position"). The keys should not make vertical rows, keys in adjacent horizontal rows must be slightly shifted, the key size is dictated by finger size and cannot be too small. The standard key placement is helpful, but also other can be learned as long other requirements are met (which, for these keyboards, are not).
Just putting letters in QWERTY order is not helpful. Probably nobody writes novels, computer software or E-mails to multiple customers on this keyboard, so limited typing speed is not much a problem.

You raise a good point, but I would like to see some sources for it. I can't see why no matter the keyboard placement or actual key placement, the design wouldn't still include QWERTY, to which most people from the early 1900s typewrite-era to modern day computer keyboard are would've been used. I definitely spend more time searching for keys in an ABCD-keyboard than QWERTY... Also why would you have two layouts anyway? Furthermore the A350/A380 both feature two keyboards per pilot that both now have QWERTY-layouts. The keyboard in place of the FMGS (A32S) is thus now also a QWERTY.

I disagree on the notion of "needing" a QWERTY keyboard in front of you.
I'm a Software engineer who learned 10 finger typing off a CD-Rom course when I was a kid. I can type blindly and from the side using a QWERTY keyboard. Why do you think F and J keys have bumps on them?
We all spend time on QWERTY because it's a very efficient layout. What is the touch-screen keyboard on your phone?
If I'm in distress, I'd rather use a layout that's familiar from everyday use.


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How much time do pilots have to execute a TCAS Resolution Advisory?
The Wikipedia entry for TCAS (Traffic collision avoidance system) states for resolution advisories (RA):
"When an RA [Resolution Advisory] is issued, pilots are expected to respond immediately to the RA unless doing so would jeopardize the safe operation of the flight" (Source).
Is there a general rule of thumb as to what "immediately" translates? How many seconds (or distance?) remain from time of resolution advisory to closest point of approach? Although RAs are usually accompanied with traffic advisories, I assume there is still some response time already calculated in...

It depends a bit on speed but your linked article has the answer

Traffic advisories are provided 35-48 seconds out and a Resolution will be provided 20-30 seconds out. Since the resolution is coordinated you only need enough time to clear the other aircraft however the greater separation the safer it will be so its advised to respond immediately upon a resolution being provided. Immediately means as fast as possible.
The FAA's TCAS II outline document states that modeling was performed assuming the pilots responded within 2.5~5 seconds

In modeling aircraft response to RAs, the expectation is the pilot
  will begin the initial 0.25 g acceleration maneuver within five
  seconds to an achieved rate of 1500 fpm. Pilot response with 0.35 g
  acceleration to an achieved rate of 2500 fpm is expected within 2.5
  seconds for subsequent RAs.

source: Stack Exchange

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What is a typical ratio of aircraft in air vs. total aircraft per airline?
While commercial airlines have a fleet of x aircraft, not all x aircraft will be in the air at any given time. Typically, several aircraft are in the air, some are on the ground (turnaround, loading, unloading, etc.), and some are in maintenance (not active).
What is a typical ratio of aircraft in air vs. total aircraft of an airline? I would be interested in airlines like Southwest, Hawaiian, Alaska Airlines, or FedEx. Can anyone provide some ballpark numbers on such a ratio?
(I assume this heavily depends on the type of airline, ratio of long- to shorthaul flights, budget vs. full-service vs. cargo vs. regional airline, number of total aircraft, route network, and obviously time of day (as some airlines do not operate flights during the night, or at least reduce flights during the night), etc.)

Well here's one way to approach it:
Excluding transatlantic and transcontinental red-eye flights, most airplanes are running between 6 am and about 10 or 11 pm and are down overnight.  Let's just say 18 operational hours per day, or 6570 operational hours per year (we could define operational hours as an aircraft "available for duty").
The average airliner flies between 2 and 3000 hours per year, so lets use 2500.
Being airborne 2500 hours out of 6570 operational hours means the airplanes are airborne about 38% of operating hours.
Allow for aircraft that are in the fleet but are non-operational because they are down for more than overnight; A checks, C checks, D checks, and also dispatch delays.  We could adjust the percentage number down somewhat, say several percent, to allow for that.
We are left with roughly (very roughly, there will be a lot of scatter no doubt) 1/3rd, or slightly better, of a fleet airborne at any one time during the operational day, if you surveyed across a national fleet.

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Why are only captains allowed to taxi the aircraft?
At most airlines, only the captains are allowed to taxi the aircraft (i.e. Lufthansa). Even if the copilot is pilot flying, the captain will taxi the aircraft to the runway, and later from the runway to the gate. What is the reason for letting only captains taxi the aircraft?

Taxiing can sometimes be one of the more difficult tasks required on a flight, and affords one of the few opportunities to run into something.
Historically, in the earlier days of crewed flying, a captain may have done most or all of the flying. A captain might have let the copilot fly some of the tedious cruise portion of the flight, and a nice captain might have even let a copilot land or takeoff. As CRM developed, the concept of both pilots sharing the flying duties was eventually standardized.
Most larger aircraft are steered in taxi primarily by means of a steering tiller. When installed, a steering tiller is always installed at least on the left side of the cockpit. In the past, aircraft did not offer an option for a second steering tiller on the right side, likely reflecting the norms of the day. Updated aircraft and newer designs commonly include as standard—or offer the option of—a steering tiller on the right side as well. Some companies choose to omit this option as a weight and cost saving measure.
Some companies allow either pilot to taxi the airplane unless the aircraft is not properly equipped to allow this. I do not have personal knowledge of any companies that forbid the first officer from taxiing, but such policies may well still be in place. In many cases it is still not an option since the aircraft is not properly equipped to be taxied from the right seat.

FWIW, back in the 1990s I flew for 5 years with a company (now out of business) that prohibited f.o.s taxiing their 747s (tillers on both sides). I thought it a foolish and counter-productive rule, and I ignored it, allowing f.o.s to taxi when it was their leg. They almost always appreciated the opportunity, and only once while maneuvering in tight quarters did I take the airplane from the f.o. for the simple reason that if we hit something, I wanted it to be me at the controls. Taxiing the 747 is, in my opinion, a lot of fun. I think we all felt that way.

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What is the maximum nose wheel deflection angle?
What is the maximum nose-wheel angle of deflection on modern commercial aircraft such as the Boeing 787? Can it deflect to 90 degrees or even more?

For the B787, the maximum steering angle is 70°. This information can be found in the Airport Planning Manual:

Source
For the A380, which has also a steerable main landing gear, the nose gear steering limit is the same:

Source
The center of the turn is off center, at the level of the main landing gear. With a multiple-boogie landing gear the main gear slips on the ground, which is not desirable.
This effect also occurs with a single bogie, unless the wheels are steerable. Limiting the steering angle also limits this slippage, and the wear going with it.

I suspect anything much beyond 70° would make the aircraft rather difficult to maneuver on the ground. Certainly a nose wheel fully perpendicular to the forward motion could become... interesting, if there is even the slightest thrust imbalance.

The maximums indicated here do not necessarily mean that is as far as they will go however... https://en.wikipedia.org/wiki/JetBlue_Flight_292 comes to mind where Airbus A320-200 landing gear stuck at 90 degrees.

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What is the difference between buffeting and fluttering?
As I understand, both are dynamic effects of aeroelasticity - however, what is the exact difference between buffeting and flutter?

Perhaps it would help if you stated precisely what you think those two terms mean, which should be easy to look up. Then, the difference will probably be fairly clear.

@ymb1 And, yet, the mouseover text for the downvote button says, "This question does not show any research effort [...]", indicating a clear network-wide policy that askers should do at least some research before asking questions. The current one-sentence question shows zero evidence of that, and also gives potential answerers little hint about what level to pitch an answer at.

The vernacular meaning is...
1) Flutter is un-commanded self perpetuating (positive feedback) destructive & adverse cyclic movement of any part of an aircraft. It most commonly occurs on a control surface but may be an entire wing, tail surface, or more rarely fuselage or other part or device. Flutter may be induced either mechanically or aerodynamically and may be sustained mechanically (i.e. engine or prop, landing gear or ground resonance of a helicopter skid) or aerodynamically by self perpetuating (positive feedback) force. Mechanical inception can be (but not limited to) brisk or pilot induced oscillation, weak, unsupported, or flimsy structures. Aerodynamic inception can be (but not limited to) undesirable cg, or aerodynamic forces on parts. Flutter is always undesirable, dangerous and a destructive force.
2) Buffeting is a turbulent stream of air striking any part of the airframe. The turbulent air may be completely external (weather) or created by the aircraft it self (prop, wing,etc). The most common form of buffeting is turbulent airflow over a wing (i.e. during a stall or wind gusts) striking the wing or horizontal tail. Buffeting is not usually considered self perpetuating with positive feedback, nor is it considered to be precisely cyclic or harmonic - though it may be repetitive. Buffeting is usually considered undesirable but benign and not a destructive force.

Flutter is characterized by a frequency neighbourhood of a structural eigenmode and a cyclic aerodynamic load. This causes their motions to be mutually reinforced.
Buffeting, on the other hand, is just the consequence of a cyclic aerodynamic load, such as an oscillating shock, or separated flow hitting another surface downstream.

Both buffet and flutter is a vibration of a surface of the aircraft, such as a wing or tail, due to the air flowing over it (especially if the air is turbulent). The difference, I would say, is;

If the surface does not exhibit an increasing amplitude, as it is naturally damped at the frequency of the excitation force, it is called buffet.
Conversely, if the surface is not naturally damped at the frequency of the excitation force, its amplitude of oscillation will continue to grow, and likely structurally fail, catastrophically, it is called flutter.

In flutter, there is a coupling between the aerodynamic forces and structural response, that needs to be taken account of, in analysis of flutter, which is not required in buffet. See the first paragraph of this technical paper.

The flutter analysis is generally conducted by complex eigenvalue analysis, whereas the buffeting response is typically estimated using a mode-by-mode approach that ignores the aerodynamic coupling among modes.

As examples of each, here is a video of flutter. As can be seen, the amplitude of vibration starts to grow, and it wouldn't appear surprising if the tail plane failed very quickly.
In comparision, the F-18 has experienced tail buffet from vortices from the wing leading edge extensions. As can be seen, the tails vibrate, but its a much smaller oscillation than the flutter example. While it could cause cracking in the long term, it would not be expected to fail instantly.

" ...due to the air flowing over it " - not necessarily... it can be induced mechanically  such as landing gear, engine or prop vibration.

@jwzumwalt. Hi. I agree a "mechanical" event such as PIO or turbulence off the landing gear could trigger the occurrence of flutter. But, in either case an aerodynamic force still results on the surface. So, as I see it, the air flowing over the surface, still plays a fundamental role. Or have I missed something? I think all answers are very similar, just a different way of saying it. Regards.

This list tends to have folks that split hairs - I believe more in a sensible approach. Its more of a FYI that there is both mechanical AND aerodynamic flutter, and combinations. Pilots usually only think of control surface flutter - but there are a number of other types. Purely mechanical with no aerodynamic input is a helicopter skid that produces ground resonance that can completely destroy a helicopter in less than 10secs. Flutter in the landing gear such as a tail wheel can tare the whole tail off an airplane - no aerodynamics there either. There are others...

@jwzumwalt.  OK, with the tail wheel example I think what your are suggesting there is a type of "shimmy". If that's the case, I now see an example that is totally mechanical. So, thanks for the comment. I had interpreted "flutter" to be only the aerodynamic type (especially as it was being compared with buffet).


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What are the triggers of the MD-80's takeoff configuration warnings?
I'm helping develop sounds for a MD80 simulation and I've noticed the MD has up to 5 alarms that fire when you advance the throttle and the aircraft is not properly prepared for takeoff:

"Beep beep flaps": I assume when flaps < 11 degrees.
"Beep beep slats": (???) Don't slats move along with flaps? Isn't it redundant?
"Beep beep brakes": I assume you have parking brakes set.
"Beep beep spoilers": I assume you have spoilers in other than stowed condition.
"Beep beep stabilizer": (???) Stabilizer trim is off for takeoff? What is the correct setting?


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What did the "gear warning horn" on the North American P-51 sound like?
I'm working on a sound project for a P51 simulation and I'm having real trouble trying to find a reference on how the P51 gear warning horn sounded like. What mechanisms were used in the '40s to make these kind of sounds?

I don’t even know if the airplane had a gear warning horn on it.

The tail warning radar just used a mini dingalingaling fire bell along with a red light on the glareshield.

Interesting question.  Unfortunately, I couldn't locate the part in the P-51D/K parts catalog, but, if an E-bay seller is to be believed, it had an E-2 Faraday horn, which appears to be an electrically actuated alarm.  Faraday is (was) the name of an alarm manufacturer most commonly known for their fire alarms.  Based on a history paraphrased from cteksys.net (emphasis is mine), this seems to be plausible:

Faraday, was founded in 1875 and is now one of the leading
manufacturers of Notification Appliances, Fire Alarm Control Panels,
and Clock Systems in the United States.  (Statement in 2002) ...
... In the 1940s Documents have been located with the name Schwarze-Faraday at 130 Church Street Adrian Michigan showing the
company pressing into signaling equipment for industrial applications
including factories , ships, and airplanes.

It's not the original, but a circa-1960s Sperti-Faraday horn (the company name from 1937-1965) sounds like this:

Thank you so much for such detailed reply! <3. Yesterday after I asked this I continued investigating and the closest I got was this: https://www.ebay.com/itm/Type-E-2-Landing-Gear-Warning-Horn-WWII-Aircraft-P51-B25-P38-P40-P47-Warbirds-/252619301037 but it seems to be made by Sparks Withington Co. According to the seller they "fit the following aircraft..." in which the P51 is included, but maybe it wasn't on the original equipment. Your reference is the closest I got so far. Many thanks again.


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In simple mechanical stick linkages, is there crosstalk between the aileron and the elevator movement?
The background story:
I'm an aviation enthusiast (who has played a lot of sim in his life and will play lots more :D) but I've never actually flown any plane myself, as in, actually used the controls (but I'm planning to make my PPL one day, I've always wanted to have wings :).
I also have an idea in my mind which involves a simply constructed stick control column.
While looking at some pictures of the mechanical construction of the linkages that translate stick movement to aileron/elevator movement, I've stumbled upon this design analysis of the Mitsubishi A6M3 Zero, a japanese WWII fighter plane.
The bewilderment:
Looking at this drawing of the linkages on the stick,

I was wondering:
If the stick is rotated exactly and only around the aileron axis (thus describing a pure 2D movement along the surface of a circle, basically, the stick does not rotate around the "bolt" (A) with which it is mounted to the bell crank), it looks to me as if this would inevitably lead to a change in angle on the elevator, since the distance between the point where the push pull rod is linked to the stick (B) and the point where it is linked to the "elevator hinge" (C) and the push pull rod would pull stronger on the elevator (leading to a higher AOA) the further the stick is rotated around the aileron axis.
The question(s):
Does this mean that in aircraft with similar stick mechanisms, the further you rotate the stick to the left or right, the more you would have to push it forward/the less you would have to pull it back in order to not change the pitch of your elevator?[1]
If this is the case, is it a problem or does one intuitively get used to compensating for this "crosstalk"?
[1]: I'm not talking about lowering your AOA the more you bank to avoid stalling. Just imagine you're in a steep bank and push the stick to the side quite far to level your wings. Would you have to push forward so you don't increase your AOA while levelling out?

I believe you're looking at a center stick design. That's why the movement of center stick is relatively small compared to a wheel; the resultant angular difference is sufficiently small that the parallelogram still mostly holds.

I see, so basically the error is there, but the length of the push pull rod to the elevator makes it relatively irrelevant?

Regardless of length, the total authority in roll axis is probably around 20deg. I would contend that this small angular shift makes the difference in pitch acids "irrelevant".

In the SportCruisers I’ve flown, any “cross-talk” between aileron movement and elevator movement is unnoticeable/indiscernible. Try looking up schematics for their center stick. It is probably as simple as the one you displayed. As far as side-stick aircraft like a Cirrus, from what I can tell, they have linkages more like a yoke controlled aircraft. IMHO from the perspective of the cockpit.

As drawn, that linkage would cause elevator movement if you moved the stick in a perfectly lateral direction.  But when you roll into a turn, you normally apply a little up elevator
anyway, and in any case the tendency is to make allowance for that kind of displacement with subtle stick movements without really realizing it.
In light aircraft it's more common to see the arrangement below, with concentric tubes; a large torque tube to run the ailerons, and a centre push/pull tube for the elevator, eliminating any geometric cross-talk because they are both running on the same axis.  An alternative arrangement is to make the big outer tube for the elevator and run it laterally, with the internal push/pull rods running to the ailerons. You see this on side by side a/c and the below pictured configuration on single seat or tandem a/c.
Airplanes with very tight space confines like gliders can't always use that tube-in-a-tube arrangement and will sometimes have separated aileron and elevator linkage tubes a little bit like your illustration, but condensed into a small space, that result in very small amounts of geometric cross talk, but it's not enough to notice when flying.

In simple mechanical stick linkages, is there crosstalk between the aileron and the elevator movement?

In almost all cases, there is no intentional-- or discernable-- mechanical coupling between aileron deflection and elevator deflection.  However in the Schleicher Ka-6 sailplane, the mechanical linkage is designed to raise both ailerons a little bit, effectively increasing washout in the wing, when the stick is far aft.  See for example page 3 in this manual.  Some other aircraft may share this unusual feature.  You can imagine why it is desirable to have more washout in a low-airspeed thermal turn than in high-airspeed cruising flight.

I think what you're missing is 'control feel' - the pilot can feel the load on the control surfaces, which is significantly larger than friction in the joints of the mechanism. You fly by the stick forces not the absolute position.
If that mechanism wasn't connected to the control surfaces, and hinge A was slightly sticky, moving the control column to the side would cause some down elevator. However in actual use, a pilot would feel the elevator input and compensate, probably without even noticing.
In fact, due to the way our arms bend from the shoulder, we would probably move the stick in an arc when we intended to move it purely sideways.
Also the designer most likely considered the cross-talk and ensured it wasn't a problem within the limits of stick travel.


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Why aren't runway numbers painted in a higher aspect ratio?
I'm watching a lot of aviation youtube channels and it seems to me that when attempting a landing on a "normal" glide path, it must be quite hard to read the runway numbers until you're on short final, which might affect the number of "wrong runway landings", especially on uncontrolled airfields.

Are they actually easier to read in reality than in the videos I'm watching?
If not, why aren't they painted with a higher aspect ratio (e.g. the numbers being twice as high, thus making them appear "less vertically compressed" when approaching on a "normal" glide path)?

Don't forget the visual runway confirmation step before the beginning of the takeoff roll. If the pilots fail to do that, that can easily lead to incidents or even accidents like Singapore 006.

Valid point. Not sure if numbers with a greater height might be harder to read in that case.

Runway numbers need to be readable from multiple angles, they aren't just for those on approach. Joining at uncontrolled airfields is usually done by flying over the airfield or approaching from any point on the compass, aspect ratios opposite to your viewing angle are hard to read and therefore counter productive for those in the pattern.

Good question!
Most incidents involving aircraft landing on the wrong runway occur when there is more than one runway in the same direction. In those cases, runways are identified as Left, Center and Right, or just Left and Right. I don't know of any airport with more than three parallel runways. There are other incidents where aircraft land on taxiways, but those are differentiated with blue lights, which makes them hard to miss.
Runway numbers are useful, but the easiest way to figure out if you are landing on the right runway is to look at the compass first, then look outside to see if you're landing on the right parallel runway if there are more than one.
Also, runway numbers are useless when the weather is obscuring the runway and you have to fly the approach on instruments.

KLAX is an example for an airport with more than three parallel runways. If there are more than three parallel runways, the numbers are one-off (as explained in this answer), which might make the identification of the rwy by looking at your heading a bit confusing (e.g. if you land on one of the 06 rwys in LAX, you still have to maintain a 070 heading on final, since the rwy headings are all 070.7. Nevertheless, the runway pairs are separated from each other by the airport buildings and there are no crossing rwys, which might make it easier.

Also, there are no crossing rwys there, which might also make it less confusing.

Ah. It's California. 'Nuff said. :)

Adding to @JanNash's mention of KLAX, especially in relation to the mention of uncontrolled airfields, there's also the issue of multiple airports close together. My home field is like that; there's another airport very close by, both with runways on the same heading. In that case, one of the airports has the runway number off by one, to disambiguate the two.

That reminds me that years ago in Puerto Rico an Avianca 747 tried to land at TJIG instead of TJSJ, even though there is only one runway at TJIG and had the wrong runway number. Made one heck of a racket and scared the beejezus out of the entire residential area.

KATL, KORD and KDFW have 5 parallel runways.  There's no significant risk of lining up on the wrong-numbered set, and you really only have to worry about L/R or L/C/R when on a visual approach rather than ILS approach.


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Do low-cost airlines make more profit than luxury airlines?
I was watching this TV show and it said that currently low-cost airlines e.g. Ryanair/Easyjet make more cumulative profits than more expensive long haul airlines such as British Airways and Virgin airlines. This confused me because long-haul flights are more expensive and from looking at the type of aircraft British airways uses, those flights can carry more passengers than a 2 propeller Ryanair flight. So can someone please tell me how low-cost airlines are making more profit than long-haul airlines!!

“2 propeller Ryanair flight”‽ Ryanair does not operate any aircraft that would have propellers!

I'm voting to close this question as off-topic, because while it asks about aviation companies, any answer would be economical and we have http://economics.stackexchange.com/ for those. The question is also somewhat unclear: we would need exact statement from the documentary, because small differences may matter.

Well, cumulative profits are merely a matter of size. If you make thousand flights and the competitor makes twenty, you will make more in total even if the competitor makes ten times more on each.

@JanHudec Ryanair operates a few ATR-42 aircraft. Technically, a "2 propeller" aircraft.  Although, for the OP's clarification, it would be more acceptable to call it a "twin turboprop".

Ryanair website: 'Ryanair only use Boeing 737-800 aircraft.'  IIRC, the ATR's were sold off long ago.

@Jimmy, no, they don't. They used to have some, but got rid of them in 1991.

It is hard to say for sure - profits will always be relative and in many cases not clear to anyone without advanced economics/accounting qualifications. However, there are a number of differences in terms of customer experience and service that can make a lot of difference to profit. Examples might be:

Crew size - it usually states that the crew on a no frills airline
are there for customer safety rather than service - on a legacy
carrier they are (technically) there for both so there are more of
them
What's included - rather than charging separately for things like
seat selection, baggage and meals, a 'frilly' (as opposed to 'no
frills) carrier will bundle them together, earning less than an
airline that charges them separately. Note - BA and a few other
airlines do charge separately for seat selection
Operating efficiency - a no frills aircraft typically spends less
time on the ground being turned around, so this expensive asset is
utilised with a higher percentage air time
Seat pitch - a smaller seat pitch means two or three additional rows
of seats in an aircraft - so greater profits per flight
Less straightforward pricing models - in many cases, a 'low cost'
airline charges more for a ticket than a standard airline, hiding
behind clever yielding of fare and additional costs added on later in
the booking process. In the travel industry I have seen time and
again people being fooled by the term 'low-cost airline'

I am sure there are many more. The operational efficiency of individual aircraft can be quickly worn away when a 'frilly' airline is required to compete with a 'no frills' on a similar route. Also, some 'flag carrying' airlines, which tend to make much greater use of joint ventures and codeshare agreements than no frills airlines, sometimes fly routes because it is a requirement of a larger deal - and if that is not very profitable (or even loss-making) it can drag down overall profit.

This answer is neither complete, nor correct. It fails to mention the actually significant operational differences (they have something to do with the uniform fleet, the type of operation or both), gives too much weight to things that are unlikely to have it (in most cases there is no difference in crew size (one FA per 50 pax is enough for service) and I doubt there is any significant difference in turn-around times either) and the explanation of pricing models is just wrong—low-cost airlines usually have simpler pricing models; they are just different.

My statements on crew size are based on my personal experience - on an Easyjet A320 I flew on a few years back, there were two cabin crew. A shorthaul British Airways flight around the same time that was either an A320 or a B737 had at least six crew for the economy section and probably another couple for the business class cabin.

Requirement is one cabin crew per 50 passengers. Unless the Easyjet flight was half-empty, it did have 4 cabin crew. You may not have met them, but they had to be there. If BA might have had more it's possible (maximum appears to be 7; there are no more seats for them), but that does not translate to other, even frill, airlines.


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Is flying bad for your health?
I've always wondered if flying is bad for your health mainly because when you fly the air pressure changes a lot and i've always thought that, that can lead to negative side affects. So can someone please tell if there are any negatives to flying which may affect your health. And are there any reasons of why flying is good for your health??

The pressure change is only bad if you already have bad ears or respiratory system. The bigger issue is cosmic radiation causing cancer and disruption to your circadian rhythm from unsociable work patterns.

I find that it lowers my blood pressure quite a bit, so I fly as often as possible.

I know this doesn't answer your question - General Chuck Yeager who probably has tens of thousands of flight hours, just turned 94 years old. :-)

@RonBeyer Have you found a doctor yet who issues a prescription for this? Would you mind recommending?

@PerlDuck See any AME who is also a pilot.

@RonBeyer I was kidding—actually I was asking for a doctor who prescribes flights as a treatment against hypertension. And in a way my health insurance will accept without getting suspicious. :-) That'd be great. I'm just a pax, but I love flying.

I know, it was a light hearted reply... If you figure out that scam let me know, honestly my hourly aircraft rate is probably less than prescribed massages for people with bad backs...

There has been some reports which have noted a higher than average incidence of certain cancers in aircrew. The reports are somewhat inconclusive

Overall, the reports suggest that pilots are at increased risk of malignant melanoma, non-melanoma skin cancer, and possibly acute myeloid leukemia, and that cabin crew are at increased risk for breast cancer and malignant melanoma. However, the question remains as to whether the observed excesses are due to occupational exposures or non-occupational factors, such as reproductive history and lifestyle

Source: British Medical Journal
Research is ongoing.

....or days spent round the pool and at the beach on those long haul layovers ;)


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What is the best method for an emergency takeoff?
(YouTube)
Before the collision at Tenerife the pilot of KLM Flight 4805 caused a 22-meter (72-foot) tail-strike by rotating early (I can't say that I blame him given the circumstances).
Is dragging the tail on the runway the best course of action in scenarios where a liftoff must happen as soon as possible?

I'd speculate that it wouldn't have helped. He actually managed to get airborne before takeoff speed by creating a high angle of attack. He just didn't get it high enough. I don't recall anyone saying they stalled before impact, so if he had kept the AoA down below tail strike angle he wouldn't have gotten airborne at all

Near as I know there are no emergency takeoff procedures and for good reason:  if you do have an emergency, stay on the ground. Period.  As for emergencies which happen during a takeoff roll, this one really isn't covered among them as there really is no emergency regiment to get airborne.  The KLM captain in the Tenerife accident made a series of judgement errors and failure to follow ATC instructions combined with bad weather and a crowded airport which contributed to the accident.  Once he was rolling, the best he could do was just hope the plane would reach Vr and climb over the PanAm jet, which didn't happen.  He panicked at the end with the tail strike, only making things worse.  But at that point he had painted himself into a corner with no escape.

Great and if you can attain that speed, so be it.  This guy could not.

@ymb1, VMU is measured by tail dragging (equipped with an auxiliary wheel for the purpose). But rotation adds drag and tail strike adds a lot more, so in normal operation, earliest lift-off would be achieved by waiting for VMU and then pulling to just below the tail-strike attitude. And if you also need to climb out, you actually need to wait a bit longer before rotating, because at VMU the wheels will leave the ground, but the plane won't climb out of ground effect.

I realize that I'm playing Monday morning quarterback but in theory he could have possibly had a better outcome by gaining as much speed as possible with wheels on the runway and then pulling up just short of a ground strike at the very last moment.  Does that sound right?

Yeah.  He could have been not so impatient to go that he ignored the PanAm 747's radio call that it had not cleared the runway during its backtaxi, adding another 40,000 lbs of jet fuel prior to departure, and worked better with his FO.  That probably would have prevented the accident from happening in first place!

Just a small point, he didn't ignore the PanAm call, he didn't hear it as there was another call at the same time.

The best way is to build speed as soon as possible.  Tricycle gear are helpful as they keep the wing at a low angle of attack prior to rotation.  Unless the field is soft or muddy, this would be the way to get airborne, perhaps with a bit of flap just before rotation.
Another very valuable technique is the rolling start from the taxiway, which carries a few more precious knots into the takeoff run.
Bottom line is one needs speed to fly.  High AOA tailscraping is not the best way for a given amount of thrust and weight.


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Why isn't the Transition Altitude depicted on SID charts?
Why isn't the Transition Altitude depicted on SID charts?
I would assume it would be a relevant information to be presented on this type of chart.

It is shown, when necessary.
But some SID procedures may not go high enough to be worried about TA. eg if a US SID only went out to say 10,000ft, then they wouldn't need TA on the plate.
Europeans tend to have a lower TA than the US, so the tendency is for the SID to climb through TA, and so it needs to be on the plate.


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Does an airfoil in potential flow produce drag?
D'Alembert's paradox states that in a potential flow, the drag force is zero on a body moving with a constant velocity relative to the fluid. But what about pressure drag? Shouldn't there be drag from the pressure distribution around the airfoil? I have heard conflicting answers from multiple sources and I would like to clear up the confusion.

Short answer: D'Alembert's paradox always holds as stated:

For incompressible and inviscid potential flow, the drag force is zero on a body moving with constant velocity relative to the fluid.

Longer answer: Pressure drag is a viscous phenomenon. Anderson section 3.18 has an extended discussion of experimental results stating that

the real flow over a circular cylinder is dominated by friction effects, namely, the separation of the flow over the rearward face of the cylinder. In turn, a finite pressure drag is created on the cylinder, and d’Alembert’s paradox is resolved.

Anderson 3.15 and any number of online resources convincingly and rigorously prove that inviscid flow around a cylinder preserves d’Alembert’s paradox (even when the cylinder is spinning and producing lift) because there will always be cancellation of aerodynamic forces due to the symmetry of the flow field across the line concurrent with the lift vector. The easiest way to convince yourself that this is also true for airfoils is the conformal mapping technique explained in this answer.


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How does one calculate the taper ratio of a compound wing?
For tapered wings, Taper Ratio = tip chord/root chord.
Is this formula still valid for compound wings or do I take the taper ratio for each section and take an average?

Source: https://www.grc.nasa.gov/www/k-12/airplane/Images/area.gif

Taper Ratio is a simple geometrical metric that is valid for rectangles and trapezoids. If the leading edge and/or trailing edge of the wing is bent or jagged, Taper Ratio does not have a very useful meaning anymore. The Taper Ratio of the Concord wing in the OP diagram is zero.

For the compound wing in the OP diagram, the situation is a bit ambiguous as well. Is the wing root the projection of the leading edge into the fuselage, and is the extension at the root just an aerodynamic streak that uses the structural members of the main wing?
The definition of Mean Aerodynamic Cord uses wing root and wing tip as well, and is subject to the same complications. There are some pretty intricate graphs for wings wing a compound planform, this one is from Torenbeek.

If you would want to use Taper Ratio with a compound planform wing, the best average would be a weighted one, where for each of the wing planform segments the Taper Ratio and the area is determined, resulting in average TR per area.


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What is the purpose of the serrated trailing edge spoilers on the B52?
At around 50 seconds on the linked video you can see the serrations on the trailing edge of the B52's spoiler.
What purpose does it have, and what gives it advantage/disadvantage over the standard spoiler? I've only seen serrated trailing edges on wind turbine blades.

I don't know for sure, but it's most likely to reduce a burble or vibration by diffusing the air flowing around the TE of the spoiler (I doubt it would be only to reduce noise).

Are you talking about these https://en.m.wikipedia.org/wiki/High-lift_device

@JohnK - I seem to recall this as well. With a straight rear edge you can get flutter and have to design for that, but the serrations spoil the resonance.

The comments above by John K. and Maury Markowitz are on the right track. The serrated trailing edge of the spoilers has an effect on vortex shedding. It is called that because it is a vortex-shaped airflow that is shed from the rear of an airfoil or other surface.
On modern jet engine nacelles the serrated edges at the rear reduce noise. However, on the B-52 the most likely reason for this feature on the spoilers is to reduce the possibility of vortex shedding inducing vibrations that could cause the wing structure to resonate due to the harmonic oscillation of the vortex. This is also known as flutter. My guess is that at some point in the life of the aircraft design it was determined that this was (or could be) an issue, and this was the solution.
If you think about the last time you flew on an airliner where the pilots deployed the spoilers while still in flight, you may remember the vibration generated by them. That is the consequence of vortex shedding, and that is what the serrated edges are designed to minimize on the B-52.


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How did ww1 pilots handle G-forces?
The wikipedia page on G-forces, https://en.wikipedia.org/wiki/G-force, lists that WW1 aircraft encountered 4.5-7g during dogfights.
This made me wonder, how did pilots in the early days of aviation handle the g-forces? G-suits were not invented until 1940s.

This interesting British document on the G loads sustainable by German WW1 fighters pegs the highest G factor for a British fighter at 5.5G for reference, with the SE5. German fighters examined and tested by the British after the war didn't fare quite as well, with max G tending to be in the 2-3 G range, beyond which the rear wing spar would fail.
What is also important to consider is that WW1 aircraft on both sides were not capable of sustaining those G's for very long, due to the low airspeed and rather underpowered engines of that day. An aircraft sustaining very high G's bleeds off quite a bit of speed in the process, due to the increased drag.
A WW2 fighter capable of 300-350 knots with a 1500-2000 hp engine, can pull a lot of g's for a long time, before scrubbing off enough speed to be in danger. A WW1 fighter at 80 knots, with a 150-200 hp engine, would only be able to sustain high G's for a few seconds, before they lost enough airspeed to risk stalling.
The few motion pictures of dogfights of that era, presumably re-enacted, tend to reinforce this: the planes appear to make a series of brief sharp turns interspersed with returns to level or near level flight, sort of a jerky movement as opposed to the long high G turns of WW2 fighters.
Finally, there were techniques some pilots used to avoid blacking out. Major Greg Boyington, who had been a boxer before the war, reported withstanding more G's longer by tightening his neck muscles up.

The document is indeed interesting. It describes calculated stresses though, not actual destruction tests.

They would use up their altitude to make up for the energy loss in turns. Von Richthofen writes in his biography that dogfights invariably ended in ground level chases after a few turns.

The phenomenon was first observed in WWI but was poorly understood. The most likely flew up to their personal limits using their body as a queue.

G-LOC of pilots, then called "fainting in the air," first occurred in
  World War I and may-have been, the cause of some military aircraft
  accidents
Head, H, The sense of stability and balance in the air, Chapter 11. In The
  medical problems of flying. H. Milford (ed.). London: Oxford
  U-siversity Press, 1920.

They may have just passed out and recovered...
According to this account, the pilot passed out  in one dog fight although in this case it may have been from either the spin or blood loss in the leg.

...but succeeded in driving down two of the enemy in a spin.
  He lost consciousness after this, and his machine fell out of control. On recovery he found himself being again attacked heavily by
  a large formation, and singling out one machine, he deliberately
  charged and drove it down in flames.

This article would make it seem like red-out may have occurred in some maneuvers but due to a lack of understanding of what was actually happening they just flew through it.

This aircraft was designed to undertake a controlled bunt, with
  structural  limits of -5G! Those pilots surely would have had “Red
  out” during those -Gz maneuvers. It could  perform a U-Turn in less
  than 200 yards of air space, clocking up  almost 8 G in the process.
  Since those brave hearts did not know anything about G-LOC, neither
  were there any anti-G suits designed then, so they flew on

It seems that little was done (or at least little was documented) during the first world war on this front as the first actual technique for G tolerance increase was not used until 1933.

A type of AGSM useful in flight was first the discovered as a method
  to increase G tolerance by Stainforth of England in 1933. He found
  that straining the abdominal muscles increased G tolerances 2 G, from
  4 to 6 G.

AGSM - Anti-G- Straining Maneuver,  Performing a isometric strain or tightening of all abdominal and leg muscles, puts muscle pressure on arteries in lower part of the body, minimizing tendency of blood to pool there in high- G maneuver, thus keeping more blood available for eyesight and brain function, and can increase G-tolerance by 1-2 Gs.

Something very interesting is that the fluid inside the eyeball is under very slight pressure (to keep eyeball inflated), and because of this, the blood pressure inside the eye is very slightly higher than the blood pressure outside the eye in the arteries which feed it. The upshot is that as G load increases, the blood pressure inside the eye goes to zero before blood pressure to the brain goes to zero. So Vision fails at a very slightly lower g-load than where GLOC occurs. Hence the "red-out" before "black out".

@CharlesBretana I understand red-out to be a reddening of vision under negative G's.  The visual phenomenon under positive G's is, I think, an increasingly narrow tunnel vision, and/or graying of vision.

@Wayne,  sorry, you are correct, it has been a while since I got out of service. "red-out" is the phenomena (seldom experienced except by aerobatic pilots) when under negative Gs. "Grey-out" is what I was referring to

Typically the human heart can produce enough head to keep the blood flowing to around +5g.  So sustained maneuvering at those limits should not pose a problem.  Proper breathing, body position and flexure of the skeletal muscles in the legs and stomach combined with physical conditioning with weightlifting and experience can allow a pilot to pull sustained loads in excess of +10g without the use of anti g suits.  Remember, at their best an anti g garment will only offer an extra g or so of enhanced tolerance.
There is also the issue of instantaneous vs sustained load factors while maneuvering.  While a WWI fighter might be capable of pulling 4-7g instantaneously, sustained loading were probably more in the area of 2-3g - early engines just didn’t produce enough power to allow for hard sustained maneuvering.  So risk of s G-LOC was minimal in early fighters.

The same article cites:

A typical person can handle about 5 g0 (49 m/s2) (meaning some people might pass out when riding a higher-g roller coaster, which in some cases exceeds this point) before losing consciousness...

So they could just pull g's until their personal limit of consciousness - natural selection would then weed out the pilots with the lowest tolerance.


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How would one calculate the thrust-to-weight ratio for a minimum turn radius?
I'm looking to calculate the T/W from the minimum turn radius while level, as well as the load factor while banking, and the wing loading W/S
I plan to plot the thrust to weight ratio against wing loading given the inputs mentioned above.

I don't think that's possible. First of all more often than not to achieve minimum turn radius specified the plane could choose to lose speed. E.g. a glider could give you a minimum turn radius number but it has a thrust to weight ratio of 0. Also even if you manage to get the minimum turn radius with constant altitude and constant speed, you can at most infer lift to drag ratio or lift to weight ratio from it, but not thrust to weight ratio, e.g. for same thrust, at minimum turn radius, thrust equals to drag, so high lift to drag ratio or low weight means small radius.

You can't. Turn radius is a function of true airspeed & bank angle, and nothing else. A J-3 Cub flying at 40 knots will have a pretty tight turn radius compared to an F-16 with its 9-G turn at 100+ knots, just because the Viper can't approach the Cub's low speed. Or take a KC-135R -- better T/W than a T-38, but not really a low-speed performer nor a high-G machine.
Turn radius in isolation will favor low-speed aircraft over high-speed aircraft, even when the latter have better T:W ratios.


text:
How do I calculate propeller effects on pitching moment?
I have read somewhere that propellers have an effect on the pitching moment of the aircraft. Having said that, I have a couple of questions.

Is this a significant effect in model aircraft? If so, are there major differences between pusher and tractor configurations?
Finally, how would one calculate this effect?

The most obvious reason for pitching moment is the propeller thrust vector is not aligned with the center of gravity. But there are other origins, explained in this article, starting page 53.

Yes there are several effects that the propeller has:

Gyroscopic precession: a pitching moment is transformed into a yawing moment and vice versa, due to the gyroscopic effects of the spinning propeller.
P-factor, which is similar but due to differences in blade angle of attack and only at high A0A of the aircraft. From the wiki:

P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,1 that is responsible for the asymmetrical relocation of the propeller's center of thrust when an aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.

These are effects that pilots of single engine propeller aircraft must cater for, and they scale down to model aircraft as well.

The propeller causes pitching (and yawing) moments. There are two main reasons for that:
1) flow change around the fuselage (and central wing and tail)
2) thrust change.
Thrust change is simpler to grasp and estimate. The thrust vector will cause a moment, because (90% of the time) it's a bit tilted and does not pass through the CG of the aircraft.
Several model aircraft have intentionally tilted propeller / motors. The perfect amount of tilt is found through flight tests, because its simpler and more accurate to try and feel the effect.
A pusher configuration has quite similar concerns related to thrust vector, however the flow change effect is much less than a tractor configuration. This is because for the pusher configuration the accelerated flow does not effect the air vehicle any more.
Calculation of these effects range from hand calculations (spreadsheets) to CFD and finally to flight tests (where you'd do controlled tests to derive the model parameters).
For RC aircraft I'd suggest following the rule of thumbs that have been established in the RC communities.
Finally, tractor / pusher configuration selection should be based on several other factors and thrust moment is probably not the most important one.


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For a plane with a pusher propeller, how does a root-stalled wing affect its performance?
My understanding is that it is desirable for the inboard part of the wing to stall first to maintain aileron effectiveness.
For the configuration shown below, if the inboard part of the wing stalls first, what would the effect be on the propeller performance? Is there an equation that estimates the effects?

Source: https://en.wikipedia.org/wiki/IAI_Heron

Interesting question. Its far easier to find studies which focus on effect of propeller on the wind aerodynamics. I highly doubt there would be any analytical equation that can give effect of stalled wing on a pusher prop. For one thing, stalled airflow goes out of the realm of what analytical aerodynamics can handle. Even for having an empirical equation, since the stall is highly irregular phenomenon, it would be very difficult to arrive at any empirical equation which can give reasonable correlation for wide enough envelop of wing-pusher configuration. The impact would be highly sensitive to geometric configuration parameters such as location of propeller with respect to centreline, distance from trailing edge and so on. For a specific configuration, one perhaps could come up with a correlation but that would need a good amount of experimental data and it would be of limited utility.
In my opinion, the impact of the stalled wing on the propeller performance is far less than the impact of the propeller on the wing stall. Definitely there would be some impact on the performance on the propeller due to "dirty" incoming airflow on the pusher propeller, but the impact should be small. I base my expectation on the fact that the streamwise velocity would be still much higher than the cross-stream velocity components in the stalled airflow behind the wing, thus giving marginal changes in AoA that the propeller blade aerofoils see. This particular research paper say this in the conclusion:

The prop-wash effect mainly increased the aerodynamic performance of the wing, but the wing–wash effect was just a small part of this interaction. However, the wing-wash effect cannot be ignored as also mentioned in previous study.

Note that this is valid only for lower Re regime for the particular configuration studies. But I would expect results not too different in other cases.

It depends on the dimensions of the plane and on the stall angle, but most (or all) of the turbulent wake produced by the stalled wing may pass above the propeller, that will continue working in 'clean' air...

You are correct. For this particular configuration that would be mostly true.


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How can leading edge separation bubbles be prevented?
I have an airfoil (S3010, 9% thickness) that gives the resulting Cp distribution at 2 degrees and Re=500k.

I suspect that the abrupt increase in pressure before the suction peak is a separation bubble. I've read somewhere that this behaviour is common for thin airfoils.
How can I modify the airfoil to prevent leading edge separation?
The end goal is to increase the L/D

The velocity-jump at $\simeq 0.5c$ is due to a laminar separation bubble.
But you ask two different things here:

Prevent leading edge separation

A leading edge separation may happen as the laminar separation bubble progresses forward for higher angles of attack and is more common in thin airfoils. A way to suppress this is by the use of roughness elements, a trip or vortex generators. You can experiment with this using Xfoil and fixing the transition location at the point (or in a distance before this point) that you assumed that you have a roughness element for example. Otherwise try modifying the geometry with Xfoil's inverse design routines and if you're seriously into this read these Low Reynolds
Number Airfoil Design Lecture Notes provided by Selig.

Increase aerodynamic efficiency ($L/D$)

Increasing $L/D$ for a specified angle of attack for these kind of airfoils and for the low-Reynolds regime, means in general to reduce the velocity-jump of the laminar separation bubble. If I am correct, Selig's airfoils are already an improved version of similar NACA airfoils for low-Re. To do this you should again consult the aforementioned notes.
Are you sure that the spike on $c_p$ plot before the suction peak is not a paneling issue? It seems unphysical. Have you tried refining your airfoil? I do not capture this with the same analysis using Xfoil with 200 panels.

A rule of thumb is that the shape parameter ($H_k$) of the boundary layer at a laminar separation bubble is around 4. You can see from the following plot that you have a laminar separation bubble around $0.5c$ on the upper surface. Not in the leading edge though.

This site lists the S3010 as an airfoil for low Reynolds numbers. This book is by Michael Selig, the person that the S3010 airfoil is named after. The book contains $C_P$ data at different Reynolds numbers, the highest being 300k. So it Looks like the OP measurement is at a Reynolds number that was higher than the airfoil was designed for, leading to turbulent boundary layer behaviour at the nose and early separation.
What is apparent from the profile drawing, is the relatively sharp nose when compared to NACA profiles of the same thickness. The obvious remedy would be to increase nose radius, round out the nose - but that would remove a design feature that all the low Reynolds number airfoils in the book have. The other option is to use an airfoil that is designed for higher Reynolds numbers.
The NACA 6-series profile listed in this question uses a leading edge radius of 1.561% c.


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What is this flat plate sticking out of the tail of Learjet 24D?
What is this protrusion on the tail of Learjet 24D (shown just above the avro museum logo) and what is its function?

Learjet 24D Tail closeup

Zoomed out image of the Learjet tail
Source: images were taken by yours truly

More importantly, how are they going to continue their tennis match now that the ball has been impaled on the wing tip?!???!?!  :)

It's a VOR antenna. You can find more information about it in similar questions like this one or this one.

Technically it's 1/2 of the VOR antenna.  There are two elements, one on each side of the tail.


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Does engine failure shut off the auto-pilot?
After answering this question it got me wondering whether there can be a situation where a complete flameout of engines could cause a serious loss of control if the auto-pilot were to continue to try to maintain altitude despite the loss of power. If it did, presumably by raising the nose and losing air-speed, eventually the aircraft would enter a stall.
Which raises the question, does the auto-pilot automatically shut off in the event of engine failure? Though I suppose it depends on the auto-pilot and the aircraft model.
I do realize the pilot and co-pilot should be very aware something is awry, but I can also see them being rather busy trying to figure out what all the alarms are telling them, and or trying to restart the engines and not notice the AP is setting them up for disaster.

I am not entirely certain whether an autopilot disconnect would be required under all circumstances, as it probably depends on the mode that the autopilot is in, anyway. It is, however, definitely an issue if the autoflight system tries to achieve flight parameters that are outside the performance capabilities of the aircraft. Airbus unfortunately has lost an A330 during flight test in exactly that way, see https://en.m.wikipedia.org/wiki/Airbus_Industrie_Flight_129.

Not always, see China Airlines Flight 006

I'm assuming you are asking about commercial aircraft? The answer is likely "no", at least it doesn't fully disengage. For example an engine failure may disengage the auto-throttle system, but roll control remains enabled. This is the nature of multi-mode autopilots where the modes can be independent. The altitude hold system may not disengage until the trim goes out of limits, so it is a secondary effect of an engine failure, but I don't think an engine failure would affect anything directly other than auto-throttle.

No, there is no necessary connection between the autopilot and an engine failing on any aircraft that I'm aware of -- which is far from all of them!  If you're in an idle descent, one motor failing wouldn't really change the flight characteristics of the aircraft, so in that case there's no great need to get rid of the autopilot.  Also, once you have everything trimmed up (i.e. rudder trim in to balance the asymmetric thrust), it's nice to put the autopilot on to let it keep the wings level & hold altitude for you while the pilots sort out everything that comes next -- checklists, ATC coordination, choosing where you'll land, etc.
What DOES come off almost immediately after engine failure, at least on the 737, is the Autothrottle.  It will sense that the thrust and throttle position are mismatched, and rather than moving the good throttle around when you may not be ready to compensate with the rudder, it disengages.  I'm not certain if that is universally true; the 777 computers are crazy smart, and they may actually leave the A/T engaged in that sort of a scenario (while the A/P flies the rudder for you).
The basic challenge that you mention, an autopilot that will hold altitude even when the thrust is insufficient to maintain airspeed, is a scenario that's not limited to an engine failure.  The automation will have some set of rules as far as when to hold, when to go into reversion modes, and so forth.  And if you get caught in the wrong spot, the automation can bite you.  In the 737, for instance, if you're in altitude hold or glideslope capture, the autopilot will hold that no matter how badly the speed decays. (Stall while on glide path, or follow the speed cursor into the dirt short of the runway... damned if you do, damned if you don't!)  On the other hand, if you're in a vertical speed mode or a VNAV climb or descent, there are speed reversions that will change your flightpath rather than accepting a speed above or below safe limits. If everything has been set up correctly and the Autothrottle is engaged (or the pilot is setting the power appropriately), then everything works really well.  But if enough things go wrong and you aren't paying attention... well, that's when bad things can happen.

I can't find a link to an article from the 90s, but there was an article where a Bonanza pilot died in flight (due to health issues), his plane came smoothly down in a field somewhere due to the autopilot keeping the wings level after it ran out of gas.
So as long as the plane has electric power, many autopilots will function when the engine stops.

If you do figure out which flight that was (there should be an investigation report, if you're willing to do some digging), you might consider adding it to the Beechcraft Bonanza Notable flights section on Wikipedia. It certainly sounds to me like it'd qualify. Neither that section nor the "Accidents and incidents" section lists anything at all from any year in the 1990s.

On most modern aircraft thrust asymmetry is compensated by the FBW irrespective of the autopilot in the following phases: takeoff(obviously since A/P is not engaged), climb, cruise, and descent; in these phases one active autopilot is managing the vertical and the lateral path, and it will keep engaged.
Another situation occurs in land mode where in most modern aircrafts we have triple redundancy(Boeing has 3 APFD computers, Airbus has 2 doubled computers), and the active computers(left and right in Boeing case)will manage the rudder too.  Normally  in land mode every AP computer has its own power supply, that is they are electrically totally separated, if the engine failure occurs while the APU is off, the A/P mode could get degraded  because of lack of separate power supply but will not disconnect, however if the APU is running, the electrical reconfiguration will prevent the degraded mode.
With respect to old generation aircraft like the B737, I can’t answer.


text:
Does the definition of visibility take object size into account?
Overall, visibility is the distance at which objects may be discerned.
Does this definition take into account the size of the objects? Obviously large objects can be seen from farther away than small ones.

@mins From an operational perspective, visibility is distinct from RVR. The two are indirectly comparable, but neither equivalent nor usually interchangeable.

@JonathanWalters, overall yes, however in some cases RVR can be replaced by visibilty assessed by pilot.

“Obviously large objects can be seen from farther away than small ones.”—that's not obvious at all, because it is not really true. Fog does not blur the image, it reduced contrast, so what matters is not the size, it is the difference in brightness. Bright lights are visible from furthest, followed by very dark objects.

The FAA gives the definition for "Flight Visibility" in 14 CFR 1.1:

Flight visibility means the average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night.

According to that definition, any mountain, geographical feature, structure, road, or other object could qualify for day visibility measurements, regardless of size. To identify the object in a way that is meaningful for measuring visibility, the object must be identified to a degree that the distance can be positively identified. If I see a mountain and positively identify it as a mountain that is at 150 NM distance, I can correctly state that I have at least 150 NM of flight visibility. Likewise, if I see a windmill and positively identify it as a windmill that is at 1 NM distance, I can correctly state that I have at least 1 NM of flight visibility.
The FAA also gives the definition for "Ground Visibility" in 14 CFR 1.1:

Ground visibility means prevailing horizontal visibility near the earth's surface as reported by the United States National Weather Service or an accredited observer.

The term "prevailing horizontal visibility" is indefinite as seen here, so additional clarification is needed. In this case, that clarification is provided by the National Weather Service itself:

Prevailing visibility is the value reported in non-automated surface observations. A human observer determines it by identifying objects and landmarks at known distances in a full 360 degree circle around the observation point. The greatest visibility observed over 50% or more of the 360 degree area is the prevailing visibility. If there is a sector of the 360 degree area that is significantly different from the prevailing visibility, the system may add a remark reflecting that difference. Pilots must realize, however, that nearly half the area around an airport may have lower conditions than the reported prevailing visibility.

Automated visibility sensors—such as those used with the Automated Weather Observing System (AWOS)—do not measure visibility over a distance. Instead, these systems sample a very small area for clarity of the air and use an algorithm to convert this value to a usable representative visibility value. For more information on these systems see the National Weather Service's Visibility Page.


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Where can I find a video about making F-16 jet fighter canopies?
I'm searching for a video of the manufacturing process for an F-16 jet fighter canopy. Can anybody help me?  Does anybody know where these canopies are manufactured?

Well, way back when the F-16 was built by General Dynamics at Carswell AFB, Ft. Worth Texas. Fun Facts: Strategic Air Command starring Jimmy Stewart was filmed there. There is a B-36 in the lake at the end of the runway. Last time I looked you could still see its tail.

P.S. There are (were?) 6 pristine F-16's at the salvage yard. Maybe you can get a replacement canopy there. These were flow directly from the plant (Carswell AFB, above) to the boneyard in Arizona. Originally destined for Pakistan, the Pakistanis did something to tick us off.

You might find your answer, or at least a good direction in the answers to this question.

Some pictures, from the name of the manufacturer provided in the answer to the question linked by @FreeMan.

@mins 1-handed, sideways, free-hand (no jig or template) routing looks like a great way to ruin a work piece and potentially remove a knee cap! (5th image down.) Yikes!

@FreeMan: Also this video. I don't know, this is hand-crafted, but the quality must be there else they wouldn't have many renowned customers. This seems to be for prototypes and unique parts, not for mass production.


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Is pitch-flap coupling an aerodynamic effect or an intrinsic property of a delta 3 offset?
I am modelling a propeller, in particularly the equations of motion of the various pitch/lag/flap angles as the propeller is turning around. The blade includes a $\delta_3$ angle, which provides pitch-flap coupling by offseting the flap hinge slightly (see image below). From what I understand, his  pitch-flap coupling essentialy reduces the flap amplitude by varying the pitch. However, is this an aerodynamic effect? Or an intrinsic effect that is a result of the kinematics.

Is this a question about helicopter rotor blades, that would not apply to a typical airplane propeller? If so, the question and title could be clarified by changing the word "propeller" to "rotor", (adding something about "rotor" to title), adding an appropriate tag, etc.  Even if the word "propeller" is not technically incorrect, most readers would benefit from a reference to "rotor" if applicable.

Short answer before I write something longer, to get a better idea: delta-3 is a mechanical coupling that utilizes the kinematics of the blade flapping to create a pitching moment that, by way of aerodynamics, stabilizes the flapping motion.  I found another answer that talks about this here: https://aviation.stackexchange.com/questions/73318/how-does-a-delta-3-hinge-on-a-helicopter-tail-rotor-work.  Is that what you're asking about?


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How does bleed air maintain pressure for pressurization after being cooled?
I found on aerosavvy.com/aircraft-pressurization the paragraph below, which talks about bleed air on airliners:

Most modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This very hot air must be cooled to a comfortable temperature before it’s directed into the cabin.

If it is cooled too much it wouldn't be under high pressure, right?
The cabin needs high pressure air because the air at 30,000ft is at too low of a pressure, so they need to simulate a 6000-8000 feet pressure. This bleed air is very hot high pressure air, I understand you have to cool it because it is way too hot for anything to operate with it.
But if you cool it, doesn't it lose pressure, so it would be low pressure air which is not what we want? I don't know where I am getting confused at.

The difference in pressure between sea level and 8000 feet is a whole 7.7" hg. At 40,000ft the outside air pressure drops to 5.5"hg (it is 29.9" hg at sea level). 1" hg == 0.49psi. Point being, the pressure's we are talking about here are very low.

@Mother Grinning Bird - your comment might make a good answer.

In the old days, when smoking was allowed on cabins, the pressurization relief valves, usually located near the rear of the fuselage, would be stained, and the paint would be stained, from the release of tobacco smoke from the aircraft. Also, there are some aircraft with are pressurized with air without benefit of an air cycle machine, however there is still thermal mixing, and ram air is still utilized with an intercooler.

The cooling process used is isobaric, not isovolumetric.  It maintains a constant pressure by increasing the density of the air by IGL:
$P = \rho *R*T$
In order to maintain a constant pressure throughout the heat exchanger, the outlet air density gets larger as the temperature drops during cooling.
This causes a drop in the volumetric flow rate through the heat exchanger but also maintains a constant mass flow rate.


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When the angle of attack is increased does the air flowing below it slow down
So the air above the wing flows faster. So it creates low pressure above the wing. The air flow below the wing, is it slowed down by the increased angle of attack or does it stay the same? So is the higher pressure on the bottom of the wing relatively higher then the bottom or does the airflow on the bottom slow down beucase if the increased angle of attack?

In high alpha flight it is not uncommon to have compression below the wing, which would increase the pressure, and act in concert with the Bernoulli Effect on the top of the wing.  The effect on the underside of the wing could also correctly be called a form of Bernoulli Effect.
There need not be any compression on the underside for there to be lift on the top side of the wing.  However in higher angles of attack there is compression, which supplements the lift.

This is an old Bernoulli explanation of how wings work that is miss-leading or just plain wrong. The air is not moving, the wing is. So to answer your question, since the air is not moving.. it can't slow down.
As the wing moves forward, the air is compressed under the wing and decompressed above it. That difference in pressure causes lift. The angle of attack changes the relative amounts of each till you reach a point where the air becomes turbulent above the wing. At that point the lift becomes erratic and drag goes through the roof.
A simple analogy can be demonstrated by taking sand or talcum powder and sprinkling it over a table or sheet of cardboard or whatever. Then take a cardboard cut-out of the cross section of a wing and sliding it flat across the sand in the normal wing like direction. The sand under the wing will clump together and form a line of sand along the bottom side of the wing. The sand above the wing will be moved away from the wing leaving a void above the wing.
Now, of course, air is not like sand, and hates a vacuum, and will rush back in to fill the void above the wing. However in order to do so it is "stretched" forming low pressure above the wing. Similarly the air under the wing tries to get out of the way and equalize itself, but in the meantime it is compressed and has a higher pressure.
It is this difference in pressure that causes a net force up through the wing to provide lift.

how is the air below the wing getting compressed and how is the air above the wing getting decompressed?

@Itzyoboi because the air has to move out of the way of the wing and as such the underside may be compressed. Meanwhile the topside of the wing forms a drop in pressure as air has to move to fill in the gap left by the wing as it passes.

All motion is relative. "Simce the air is not moving, it can't slow down." It can be pushed backwards before the wing arrives, which it does.

@Koyovis yes indeed it does. But that is not the same as the classic Bernoulli model either.

The old "Bernoulli" explanation is a poor explanation, but not for the reasons stated here. The alternative explanation (using sand as an example) is an even worse explanation, debunked many years ago.


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Why are there curved blades on jet engines?
On the fan and comssorssor and turbine for that matter there are blades and they are also curved. Im already confused beucase the fan has blades but wouldn't it blow out air not suck in air? Why are there blades even on the compressor and turbine and why are they curved?

Are you asking why they're curved specifically (which involves aerodynamics) or why they're there at all (which would imply a question about how a jet engine actually works)?

"fan has blades but wouldn't it blow out air not suck in air"... One side of the fan sucks, the other side blows. If you were standing behind the fan, it would be blowing... Same with household fans, spray a mist behind the fan and you'll see it sucks air in and blows it out the other side.

What do you mean by curved? Are you referring to the twist that each blade has from the root to the tip where the angle at the root is greater than the angle at the tip of the blade? Or are you referring to the curve of the blade that makes it look like a little wing--the camber of the blade?

@RonBeyer but like a household fan if I spray a mist in front of it it will be blown away so I get confused by the fan of an aircraft, wouldn't it blow air away from it, why does it get sucked in and not blown out?

@Itzyoboi I'm not sure how else I can explain it, one side of the fan sucks, the other side blows. They mounted the fan such that the "blow" side is towards the engine, which means that when you look at the engine, you are looking at the "suck" side. If you could stand on the other side of the fan (inside the engine) you would feel quite a wind blowing at you from the fan...

@RonBeyer I see. So why are there blades on the compressor, does it help create heat by causing more collusion with air molecules beucase it's spinning ? Does it help the same just blows the air faster?

The compressor uses blades to compress the air, the fan on the front doesn't really compress it. Heat is a side-effect of compressing a gas. See How do jet engines work?.

@RonBeyer what is the need of blades on the compressor if the fan is blowing air through a small duct channel which gets smaller?

You should look up how the compressor stages work, I really can't explain it in the comments of an unrelated question.

@RonBeyer ok and is the shape of the fan and foward swept tips why the big front fans suck and not blow? Last question, and thanks.

No, the forward swept tips deal with the efficiency of the fan, if you wanted the fan to blow out the front, all you would have to do is run the engine in reverse. Same with your home fan too, you can make it blow out the back and suck in the front if you could reverse it

@RonBeyer so no one really knows why the fan sucks insteads of blow. You said it was the position, how would that effect it? Or does it suck becuase that's just how it works

Yes, they know exactly why it sucks instead of blows. Same reason propellors blow backwards, it's designed to do that. It deal with the angle of the blade and the direction of rotation.


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What is the airflow across and below an elevator?
So when an airfoil creates lift. The air above the foil moves faster. This creates lower pressure in tip of the wing and relatively speaking higher pressure under the wing this create lift l. Along with meetings 3rd law of motion. You know it deflects aid downward the reaction is forcing the wing up. But how did this work with an elevator to my understanding. When the elevator goes up so the airflow across the top speeds up, so this creates low pressure. And the air below the elevator is relatively speaking higher pressure. But this would cause the elevator and tail of the plane to go down along with the pitch of the plane. But in realty when the elevator goes up,  the tail of the plane goes down and the pitch goes up. Same with when it goes down the the airflow below the elevator speeds up and become low pressure and the air above it is higher causes the tail of the plane to go down and the pitch up. But in reality when the elevator goes down the pitch also goes down. Really confusing, someone please help.

No the elevator creates lift to the other direction, it pushes the tail down not up. It's a wing rotated upside down. You can think the h stabilizer being a wing and the elevator as flap.

"When the elevator goes up so the airflow across the top speeds up" - it does? Where did you read that? I would have expected the opposite.

@TannerSwett well when the angle of the elevator goes up, the air above it can't move out of the way in time so it become constricted and squeezed so it moves faster which creates lower pressure above the wing so higher pressure below the wing so the net force is up lifting the tail of the plane and dipping the nose of the plane

If the last comment were true, you could create positive lift from the wings of an aircraft by attaching them to the fuselage with the trailing edge much higher than the leading edge. Wings do not work that way.

@Itzyoboi Well, that's an interesting chain of reasoning, and I don't understand aerodynamics well enough to say where it goes wrong. But anything which pushes air away from itself (as the top of an up elevator does)  is going to experience higher pressure, and anything which pulls air towards itself (as the bottom of an up elevator does) is going to experience lower pressure. Newton's laws tell you that much.

When an elevator moves (trailing edge) up, it partly moves in the way of the airflow. That means pressure goes up.The elevator is an airfoil itself, in the shade of another airfoil. If it moves up, the same thing happens when a wing moves trailing edge up: lift works downwards.
Image source
Picture above shows an airfoil with trailing edge down: lift is upwards.

Upper aft side of the airfoil: air only moves faster if the surface moves out of the way of the stream, if the shape moves away gently enough the air still tries to follow. Note that the nose of the shape had pushed air out of the way, so there is less air along the upper aft side of the picture: lower pressure.
Lower side of the airfoil: air tries to stream like it did before, but now there is a shape in the way of the airstream. The air needs to stream through a narrower stream tube: higher pressure. When the stream bumps into the airfoil at a shallow angle, it gets deflected downwards: impulse lift.

Stick your hand out the window of a moving car.  Think of your hand as the elevator.  Now twist the trailing edge of you hand up and down. Observe what happens in each case.
Think of the horizontal stabilizer as just a piece of metal to mount the elevator. The elevator is the 'working' surface.
On aircraft where the whole stabilizer moves, it is acting as a wing.

My bad. Hold your hand out, fingers closed, parallel to the ground. Twist the whole hand palm facing the airflow and then back of the hand into the airflow.

It seems very difficult to me to guess what the speeds of the air flow
will be at various places on the top and bottom of an airfoil
just by looking at the shape of the airfoil, as you appear to be
trying to do.
There are ways to calculate approximate speeds of the air flow
using advanced mathematics,
but of course that is not especially easy either.
A much easier way to think about what kind of lift you will get
(positive or negative) is to look at the trailing edge of the airfoil.
Unless the airfoil is stalled, the air flowing off the top and
bottom of the airfoil tend to continue flowing in about the same
direction, at least while they are still near the airfoil.
On the other hand, the air flow in front of the airfoil
comes from the direction the aircraft is flying.
Look at the difference in directions coming in and going out,
and this will give you an idea of which direction the reaction force is in.
This will not tell you the magnitude of the reaction force;
that depends on how much the air is deflected at points farther
above and below the airfoil. For the magnitude of the force,
you still need to do the advanced mathematics or
observe the actual force on an actual airfoil as air flows past it.

If you say that when you are trying to go up and the elevator goes down as the same time the nose try to goes down ( nose down pitching tendency) it came from the thrust line. The thrust line is an imaginary line that criteria moment around lateral axis if an airplane. You can find it in privet pilot manual book in chapter 1 section D.


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What is Engine Pressure Ratio?
I kind of know what it is but I don't fully understand it. Wikipedia says,

the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?

Related: Points of measure in a turbofan. EPR is more dependent on P2/P1 than P3/P0. Source.

The wiki quote refers to total pressure at intake and outlet. Total pressure is defined as static pressure $p_s$ and dynamic pressure $p_d$ which for incompressible flow = $\frac{1}{2} \cdot \rho \cdot V^2$.
So the Engine Pressure Ratio is
$$\frac {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{exhaust}} {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{freeStream}}$$
The turbofan exhaust gas is a mixture of the jet exhaust and the bypass air exhaust.

Yes.  It's the ratio of the pressure of the exhaust gases just before exiting the diffuser to the pressure of the air entering the intake.  On high bypass turbofans this measurement is taken from the fan air just before exiting the cold diffuser.

The big question is how do you physically measure the thrust of a jet engine?
Thrust can be thought of as the pressure that the engine produces to force the airplane forward. So if you measure the pressure at the back of the engine, and allow for the pressure at the front of the engine offsetting it, then you end up with something which can be used as a measure of the thrust, and thus used to manage the engine.


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Diversions because of fog: What are the reasons?
It's fog season again around my local airport MUC (EDDM), and I have noticed a number of diversions recently. Some planes were holding a bit before attempting to land, performed a go around and then diverted to some of the nearby airports.
It appears that mostly smaller planes (< 200 seats) from not-so-distant departure airports diverted and most long-haul flights made it to the terminal as scheduled, maybe with a bit of delay.
This made me wonder what the specific reasons for the diversions could have been, and I am asking mainly for answers providing some of the top reasons, including the background information.
The airport allows for ILS CAT III approaches, which, in the case of CAT III C, translates to auto-land, possible even in no-visibility conditions, if I understand the details correctly.
This, depending on the equipment installed on the aircraft, would mean that planes capable of performing CAT III b or CAT III c approaches could land when the runway becomes visible 75 ft above ground, and all others couldn't? Would long-haul aircraft mostly be equipped with auto-land, while regional jets probably wouldn't?
Would ATC have to use bigger separation, reducing the airport's capacity, and it would be the smaller planes who lose the game of who-gets-chosen? This would, however, most likely not explain the above example of a flight actually trying the approach and diverting only after having done so?
Those who are familiar with the particular airport might be aware that there's a long debate going on about whether or not a third runway should be built. In what ways would (or wouldn't) a third runway help prevent diversions on foggy days? My guess is that CAT II planes would have to divert no matter how many runways were built, but maybe separation issues (in the air/on the ground) would be less of a trouble?

Regarding CAT III C, note that landing is not taxiing. You might be able to autoland down to zero altitude, but you are still going to need to vacate the runway (and get to the gate) after landing.

Also important to understand is that CAT IIIC requires 3 things: adequate airport equipment, adequate aircraft equipment and adequate pilot certification, Even if the airport and given aircraft are equipped with CAT IIIC, the given crew must be qualified to perform CAT III (B and C) approaches

From http://fsims.faa.gov/wdocs/8900.1/v04%20ac%20equip%20&%20auth/chapter%2002/04_002_007_chg_0a.htm: "Special flightcrew qualification and training are also required to ensure that the aircraft is operated with the required degree of precision during these operations." Also CTRL-F for "qualification" on this page for more info.

Just as a note, I don't think CATIIIc has ever been implemented anywhere in the world. Last year the FAA was considering removing it from the regs completely.

@TomMcW Thanks for pointing out this detail. Re-reading wikipedia, including the footnotes below the table listing the different ILS categories, I can kind of understand the reasons... "a plane would have to be towed to clear the runway"

@DeepSpace This sounds like a good answer instead of a comment.

CAT3 certification is not general purpose. It is only valid for a specific airport and the crew must be current(in practice). Cat 2 and 3 must also be specified in the airline's own operational specifications.


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Why is it forbidden to wear glasses when leaving the plane during an emergency?
On my last flight with Iberia I noticed in the safety guidelines that you are not allowed to wear glasses during an emergency.

If I have to take off my glasses I would have serious problems seeing what is going on. So this makes no sense to me.
Why would I have to remove my glasses?

I think it might be referring to sunglasses, but I agree with you that it is a confusing depiction

I once asked service staff since I also need glasses to actually see something. They told me it refers to reading glasses, sunglasses and any others you don't actually wear all the time - simply because people would (a) waste time searching, thus delaying their own evacuation and obstructing others' and (b) sun/reading glasses not worn but attached to clothes could break and injure the wearer, bystanders or damage equipment.
That's at least what I was told.

Rightmost picture... Pull at a woman's skirt? What is that arrow supposed to point at?

@mins If she stays low, she can avoid the automatic machine gun fire in the aisle.

@mins Avoiding smoke in a burning plane.  See the yellow puffs of smoke above her.

@Sanchises the location of the door release latch.

This is because they don't want people holding up the evacuation by searching for their glasses.   It does not mean you are not allowed to bring glasses if you are wearing them but passengers blocking the aisle to search for their glasses would slow the evacuation and risk lives.   They do want you to actually take off your high heels and leave your luggage because these can actually slow you down.   This should be portrayed more clearly.


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Why can't we extract more from exhaust gases in gas turbines?
It is seen that EGT are of the order of 600°C (What is a normal EGT range of a jet engine?) for aviation turbines and nearly the same for stationary turbines. Why can't high temperature exhaust gases are further harnessed like adding another stage in turbine section that extracts more energy?
In aviation turbines can't we just add another turbine stage like another low-pressure turbine mounted on another spool? For land-based gas turbines we can somewhat utilize these high temperature exhaust gases in Combined cycle but what about aviation engines?

Welcome to [aviation.se]! A general rule about aviation questions: Almost any question you ask starting with "Why can't..." will be answered with "Well, you can but it isn't done because...". Your best bet is to just ask "Why isn't..." or "Why don't we..."

It can be done, but the better specific fuel consumption does not compensate for the extra weight. We all know that aero engines have to be as light as possible. In land, stationary installations, where weight matters but little, regeneration, intercooling, and even combined-cycle (in electricity generation, using a gas turbine that drives an alternator, but with the exhaust gases generating steam for a steam turboalternator, for example...). The efficiency is very, very high, but the weight and bulk are considerable, too...
But it has been tried: https://airandspace.si.edu/collection-objects/allison-t78-2-turboprop-engine
https://www.flightglobal.com/pdfarchive/view/1963/1963%20-%201880.html

@ymb  Different to what...?   Part of the thermal energy of the exhaust gases is recovered and used to improve the sfc... It answers the question 'Why can't we extract more from exhaust gases in gas turbines?'...   Of course we can, and there are engines where it's done...

From Flying, June 1962   https://books.google.es/books?id=bHZ-ngYi4zUC&pg=PA44&lpg=PA44&dq=allison+regenerative+turboprop&source=bl&ots=Tm8E0GBJRU&sig=uTRugPL_DiQTtVT9fd5dpTxo-kA&hl=es&sa=X&ved=0ahUKEwjjquOCybXTAhWFuBQKHd2kAugQ6AEIWjAI#v=onepage&q=allison%20regenerative%20turboprop&f=false

Another scheme, from 'Flight': [img]http://i.imgur.com/txT9gig.jpg[/img]

@ymb1: You can't extract all the energy in the stream.  That would violate the second law of thermodynamics.


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If I put two ductfans behind one another, would I got the sum of the rated thrust or other thrust values?
I am trying to design a VTOL system for a drone and I don't want to have extending motor arms that make it look like a quadcopter. I want to use duct fans and thrust vectoring nozzles. However, the aircraft is 25 kg and the drag caused by vertical takeoff is 2 N. But most of the RC duct fans I see online can only deliver 2 to 3 kg of thrust. So I am thinking whether I can put two ducted fans behind one another, but how much more thrust can I get? will it be the sum of the two motor, or other numbers?

How did you come up with 2N drag for vertical takeoff?  But forget about that how would you lift 25kg even if you had 6kg of thrust?

I use ansys and put in 1 m/s vertical climbe rate. And there are going to be a lot of duct fan combo

It should work but the aft fan won't be very efficient. The aft fan blades needs to have a much high pitch in order to be efficient in the high speed flow.

But if you want a 25kg aircraft to vertically takeoff, and you only have 2-3kg of thrust per fan, then you have to stack up 10 fans?

No, two fans stacked in series will not produce the same static lifting force as two fans in parallel. Several factors will be in play.
First is that by default the ducted fan blade angles will be assuming intake air at low speed. If you want to add a second stage it needs to have an a higher angle of attack on the second stage (something like the changing slope of a single fan blade but stretched across two fans). This can be partially achieved by increasing RPM but that moves away from the optimal power point of the motors unless major re-design is done.
The second is that the cross sectional area needs to change. If you have one fan capable of pulling 1 cubic meter if air through it's intake per second and pushing it out at 1 meter per second, it is moving one cubic meter per second. If the following stage is increasing that to 2 meters per second, it is trying to pull 2 cubic meters per second from the first stage! Hence the taper seen in multi stage turbines.
The third is drag through the combined duct. Drag goes up with square of speed, so the details of the surface finish and motor supports starts to matter far more than the original designer intended, so for example helicopter design.
Blades also work best in undisturbed air, and the airflow going into stage two or three of a stack will not be that without carefully designed stators between stages.

Don't forget the induced power. Drag grows with square of speed, but of the blades, and that initially only slowly when the orbital velocity is much greater than the flow velocity. But the induced power, due to energy given to the stream, grows with the flow velocity (just linearly) from the start.